Grand Canyon Formation: Evaluating the Overflow Mechanism

Introduction and Hypothesis Overview

Grand Canyon formation was explored by evaluating four possible hypotheses: superimposition, antecedence, piracy, and overflow. Initially, the superimposition mechanism intuitively seemed to be the more plausible explanation, but the weight of the evidence from the field trip supports the overflow mechanism. Though the overflow mechanism still leaves some questions unanswered and some formations and processes unaccounted for, it offers the best overall explanation for how the Grand Canyon was formed.

Establishing the Age of the Colorado River Relative to the Kaibab Plateau

The first requirement for explaining Grand Canyon formation was determining whether the Colorado River was older or younger than the bedrock high of the Kaibab Plateau. Based on information provided at the various field stops, one can conclude that the Colorado River is younger than the Kaibab Plateau.

At stop 2e, there is evidence showing that the topography of the bedrock high controls the location of the transverse drainage incision into low-elevation saddles. The topographical map shows how Marble Canyon is controlled by fractures. As for the mystery of Grand Canyon formation, one would ordinarily expect water to flow away from an upwarp; however, the Colorado River and Little Colorado River, with the excess energy provided by the overflow transverse drainage mechanism, were able to flow up to their confluence.

One of the stops that shows evidence of sediments downstream of the bedrock high — sediments that record the rapid arrival of transverse drainage — is stop 9d. The gravels in the two photos were deposited by the Colorado River above Lake Mead, which is downstream from the Kaibab Plateau. These gravels are termed exotic because they are unrelated to the rocks with which they are now found at the Lake Mead location, and have been transported from their place of origin by the drainage process. Stop 10a also refers to the existence of downstream sediments, which support the thesis that the Colorado River is younger than the bedrock high.

Field Evidence for Regional Drainage Reorganization

Stops 3a, 3c, 5c, and 7a show evidence of regional drainage prolongation or reorganization. The Mazatzal Mountains were once connected with the nearby ranges to the east and west in a formation called the Mogollon Highlands, until extensional faulting occurred approximately 25 million years ago. Once the Mogollon Highlands stopped draining onto the Colorado Plateau, the drainage switched from flowing to the northeast to flowing southwest. Stop 3a also describes interior drainage — closed basins with lakes — as being common on the Colorado Plateau before an integrated Colorado River began to flow, providing another indication that the overflow mechanism was responsible for Grand Canyon formation. Stop 7a shows the unusually large tributary canyon, Bright Angel Canyon, which runs straight because Bright Angel Creek erodes headward along the fault plane, exploiting weaknesses such as faulted rock.

Stop 5c, showing the Tapan Flow, also discusses the rate at which the Little Colorado River has been cutting into its channel. According to studies by Holm (59–63), if the Little Colorado River originated by integration of the Bidahochi basin and the Colorado River, then average incision rates imply a 6 Ma age for the eastern Grand Canyon where the Little Colorado River enters it. This estimate argues against both superimposition and antecedence, both of which require that the Colorado River be older than the Kaibab Plateau.

The evidence gathered at the preceding stops argues convincingly that the Colorado River is younger than the uplift of the bedrock high, which in turn disqualifies superimposition and antecedence from further consideration. Only one stop, 5b, appeared to support the piracy mechanism as being responsible for the transverse drainage. By contrast, evidence at ten stops describes processes attributable to an overflow mechanism.

Conclusion

Not all evidence encountered during the field trip supports the overflow mechanism, nor does it explain all of the geomorphic processes that were discussed. For example, superimposition provides a better explanation for transverse drainage incision associated with a regional drop in base level and prolonged denudation, as seen at stops 2f and 5d. A piracy mechanism is a better fit for explaining a possible different paleo-flow direction for the transverse drainage upstream of the bedrock high. As this and other papers and research have shown, there are still many unanswered questions about Grand Canyon formation.

Works Cited

Holm, Richard. "Pliocene-Pleistocene Incision on the Mogollon Slope, Northern Arizona: Response to the Developing Grand Canyon." N.d. Web. 24 June 2011.

Spencer, Jon, and Philip Pearthree. "Abrupt Initiation of the Colorado River and Initial Incision of the Grand Canyon." Arizona Geology 35 (2005): 1–6.

Treiman, Allan. "Grand Canyon — The Great Unconformity." Lunar and Planetary Institute, 23 Sept. 2003. Web. 24 June 2011.